1. Field of the Invention
The invention relates to orthopedic joint replacement and, more particularly, to a prosthetic device for use in orthopedic joint replacement and a system for preparing a bone to receive the prosthetic device.
2. Description of Related Art
FIG. 1A illustrates the bones of a hip joint 10, which include a portion of a pelvis 12 and a proximal end of a femur 14. The proximal end of the femur 14 has a superior aspect 14a and an inferior aspect 14b and includes a ball shaped part called the femoral head 16. The femoral head 16 is disposed on a femoral neck 18, which is connected to a femur shaft 20. As shown in FIG. 1B, the femoral head 16 fits into a concave socket in the pelvis 12 called the acetabulum 22, forming the hip joint 10. The acetabulum 22 and femoral head 16 are both covered by articular cartilage that absorbs shock and promotes articulation of the joint 10. Additionally, as shown in FIG. 1C, the superior aspect 14a of the proximal end of the femur 14 includes a vascular region 24 having a high concentration of retinacular vessels 24a located near or on the surface of the bone of the femoral head 16 and neck 18. The retinacular vessels 24a supply blood to the bone tissue of the femoral head 16.
Over time, the hip joint 10 may degenerate (for example, due to osteoarthritis) resulting in pain and diminished functionality. To reduce pain and restore functionality, a hip replacement procedure, such as total hip arthroplasty or hip resurfacing, may be necessary. During hip replacement, a surgeon replaces portions of a patient's hip joint 10 with artificial components. In conventional total hip arthroplasty, the surgeon removes the femoral head 16 and neck 18 (shown in FIG. 2A) and replaces the natural bone with a prosthetic femoral component 26 comprising a head 26a, a neck 26b, and a stem 26c (shown in FIG. 2B). As shown in FIG. 2C, the stem 26c of the femoral component 26 is anchored in a cavity that the surgeon creates in the intramedullary canal of the femur 14. Similarly, if the natural acetabulum 22 of the pelvis 12 is worn or diseased, the surgeon reams the acetabulum 22 and replaces the natural surface with a prosthetic acetabular component 28 comprising a hemispherical shaped cup 28a (shown in FIG. 2B) that may include a liner 28b. In cases where the acetabulum 22 is healthy, the surgeon may leave the natural acetabulum 22 intact and replace only the femoral head 16 and neck 18.
In contrast to total hip arthroplasty, which is highly invasive, patients who have healthy subsurface bone and disease that is confined to the surface of the femoral head 16 may be candidates for hip resurfacing. In conventional hip resurfacing, the surgeon removes diseased bone from the femoral head 16 using a rotationally symmetric cutting tool, such as a cylindrical reamer 30. As shown in FIG. 3A, the surgeon centers the cylindrical reamer 30 on an axis A-A defined by a guide hole G created in the femoral head 16. In operation, the cutting element of the cylindrical reamer 30 rotates about the femoral head 16 cutting away diseased surface bone and resulting in a femoral head 16 having a rotationally symmetric surface shape 16a. As shown in FIGS. 3B and 3C, the reamed femoral head is mated with a prosthetic femoral head cup 32. The femoral head cup 32 typically has an internal surface shape that substantially corresponds to the rotationally symmetric surface shape 16a of the reamed femoral head so that the cup 32 will fit securely in place. The femoral head cup 32 also includes a central stem 32a that is received in the guide hole G to aid in alignment and stability of the femoral head cup 32. As with conventional hip arthroplasty, hip resurfacing may include replacement of the acetabulum 22 when the natural acetabulum 22 is damaged or diseased.
As can be seen by comparing FIGS. 2C and 3B, hip resurfacing is less invasive and preserves more bone than conventional hip arthroplasty because only a portion of the femoral head 16 is removed leaving the femoral neck 18, the subsurface bone of the femoral head 16, and the intramedullary canal of the femur 14 intact. Although conventional hip resurfacing removes less bone than conventional hip arthroplasty, the procedure still removes a significant portion of the femoral head 16, including healthy bone. As shown in FIG. 3D, one consequence of the conventional resurfacing process is that the bone cuts can impinge upon the vascular region 24 of the femur 14 resulting in damage to the retinacular vessels 24a. This damage adversely impacts the blood supply to the femoral head 16, which can ultimately lead to necrosis of the bone, loosening of the implanted femoral head cup 32, pain, and femoral fracture. Additionally, if the cylindrical reamer 30 is undersized or malpositioned, there is a danger of the cylindrical reamer 30 contacting the femoral neck 18, creating a notch in the femoral neck 18. This femoral “notching” causes a stress riser in the femur 14 that increases the risk of femoral fracture, particularly if the notching occurs on the superior aspect 14a of the femoral neck 18, which is in tension during activities such as standing, walking, and running.